Signal transmitting cable

ABSTRACT

A cable ( 302 ) has (8) fibres ( 304 ) are encapsulated by a UV curable layer ( 306 ) having a diameter of approximately (1010) microns, and (16) outer fibres ( 316 ) arranged in a circular formation around the inner fibres ( 304 ). The optical fibres ( 304 ) are held in position by means of the UV curable layer ( 306 ) so that the UV curable material of the layer ( 306 ) does not penetrate into the gaps between the optical fibres ( 304 ) and the outermost optical fibres ( 304 ) are restrained by the layer from moving axially. It is found that such an arrangement provides surprisingly favourable bending properties, making the cable particularly suitable for installation in a tube by means of blowing.

The present invention relates to signal transmitting cables, and relatesparticularly, but not exclusively, to optical fibre signal transmittingcables.

Optical fibres have traditionally been installed into underground ductsby attaching a pulling member to one end of the cable, and winching thecable into the duct. As a result, such cables were large and heavilyreinforced to protect the relatively delicate optical fibre elementsfrom damage during installation.

Traditional cables were constructed by first manufacturingsub-assemblies comprising tubes manufactured from thermoplasticmaterials and containing typically twelve fibre optic elements. A numberof these tubes were then assembled together by stranding them around acentral strength member. The stranding process, and the fact that thetube is large relative to the space occupied by the fibre opticelements, means that all fibres experience the same strain when thecable is bent during installation, and the loose tube constructionallows the fibres to move and accommodate the strain, resulting inminimal signal losses.

More recent techniques for cable installation involve blowing the cableinto a duct by means of compressed air, for example as described in EP0108590. This blowing process distributes the installation force alongthe entire length of the cable within the duct, as a result of which theinstallation force at the leading end of the cable can be reduced, andmuch of the reinforcement can therefore be removed from the cable. Thisprovides significant advantages, since there is an increasingrequirement for cables to become more compact, primarily because citynetworks are congested and providing new underground ducts in cities isexpensive and involves substantial disruption.

Installation of cables by blowing involves both the use of fluid dragoperating on the sheath of the cable, and a pushing force, usuallygenerated by drive rollers or a caterpillar pushing device which formspart of the blowing equipment. At the initial stages of installation,there is very little cable installed in the tube, and the effect offluid drag is therefore small compared to the pushing effect. As more ofthe cable is installed, the installation force derived from the fluiddrag becomes more significant.

It is therefore desirable for cables designed for installation byblowing to have adequate stiffness to facilitate the initial pushingrequirement. In the case of cables constructed from sub-assemblies, thefibres are loosely contained in an outer sheath of the sub-assembly.Because the individual fibres are not constrained, they do not providethe cable with sufficient stiffness, and it is therefore desirable thatthe cable be constructed with a central strength member, typicallymanufactured using a glass-reinforced polymer. The strength member issufficiently stiff that it dominates the stiffness of the assembly and,because of its central location, ensures that the cable does notpreferentially bend in one direction rather than another.

However, the use of a central strength member undesirably increases thesize of the cable.

An attempt to produce a cable for installation by blowing without theuse of a central strength member is disclosed in EP 0521710, whichdescribes a cable in which 2, 4 or 8 individual optical fibres are intouching contact and are encapsulated in an outer layer, typically a UVcured acrylate. Encapsulation of the fibres in a UV cured acrylateresults in the individual fibres being restrained from moving relativeto each other, and the cable derives its stiffness from this,eliminating the requirement for the central strength member. However,the fact that the fibres are locked together means that when theassembly is bent, the fibres impose a strain on the outer coating of thecable. The larger the diameter of the fibre unit, the greater thetensile stress applied to the outer surface for a given bend radius.Fibre optic cables containing 4 or 8 fibres are found to create such ahigh load that a phenomenon known as fibre breakout is experienced, andwhich has a detrimental effect on cable performance.

EP 0521710 discloses a process which produces satisfactory results oncables with fibre counts of 2, 4 and 8 fibres by changing the coatingarrangement to ensure that fibres do not break out of the coating, evenwith larger diameter cables containing 8 fibres. However, it isdesirable to manufacture cables having more than 8 fibres, but attemptsto manufacture such cables have had difficulty in overcoming the problemof fibre breakout. An attempt to overcome this problem is disclosed inEP 0422764 in which 12 fibres are provided, the fibres being accuratelylocated and locked in position relative to each other by firstassembling sets of 4 fibres into a ribbon sub-assembly by edge bondingthe 4 fibres to each other, and laying 3 such sub-assemblies on top ofeach other to form a basic construction which is then encapsulated in anouter layer.

Compact ribbon cable assemblies of this type suffer from the drawbackthat the surfaces of the ribbons in such cables are smooth, and theribbons are therefore free to slide relative to each other. In addition,because the fibres are bonded in a flat arrangement, when the cable isbent in a direction which imposes a sideways moment on the flat ribbons,the force generated is high and the central ribbon, which is free toslide between the two outer ribbons, is then forced to break out throughthe outer acrylate coating, producing micro bending and unacceptablesignal losses.

An attempt to overcome this problem is disclosed in DE 4211489 byreducing the diameter of the individual optical fibres. An individualfibre is provided with a protective outer layer of 25 microns or less,instead of the 60 micron coating usually applied. This reduces theoverall diameter of the individual fibres by approximately 30%, whichhas the effect of making the assembly smaller and therefore reducing thestrain imposed on the coating. However, this arrangement is inconvenientbecause most commercially available fibres have the same dimensions, andequipment for splicing and terminating fibres is therefore adapted tothese standard dimensions. Furthermore, DE 4211489 describes anarrangement in which adjacent fibre ribbons are offset to reduce theheight of the assembly. Such ribbon constructions produce assemblieswith a very high preference to bend in one direction, and are thereforenot suitable for cables designed for installation by blowing.

U.S. Pat. No. 5,787,212 discloses an arrangement of 7 fibres of equaldiameter in which 6 fibres are disposed in a circular pattern intouching contact with each other and around a central fibre. When thefibres are coated with resin curable by UV radiation, the touchingfibres ensure that resin does not enter the spaces between the fibres,which minimises the problem of UV light not adequately penetrating theouter fibres and inadequately curing resin located between the fibres.Uncured resin has the potential to break down and generate agents whichmay damage the glass fibres, adversely affecting their long-term signaltransmitting performance.

Although the arrangement of U.S. Pat. No. 5,787,212 has very goodbending properties, since it is completely balanced with no preferentialbending characteristics, and strain imposed on one fibre is partiallydistributed into the other fibres by virtue of the touching contact,groups of 7 fibres are not used commercially, since fibres are almostalways deployed in pairs and it is desirable to manufacture cables withhigher fibre counts suitable for installation by blowing. Traditionalcables almost exclusively contain 12 fibres or multiples thereof.

Preferred embodiments of the present invention seek to overcome theabove disadvantages of the prior art.

According to an aspect of the present invention, there is provided asignal transmitting cable comprising a first signal transmitting portionincluding a plurality of elongate, flexible first signal transmittingmembers, wherein the first signal transmitting members are surrounded bya first layer of resin material curable by means of radiation such thatonly the outermost signal transmitting members are in contact with saidfirst layer, and said first signal transmitting members are arranged toform at least three rows, wherein for each said row containing aplurality of said members, said members are arranged such thatneighbouring members of a row are in touching contact with each other,each recess formed by neighbouring members of a first said row facingtowards a second said row accommodates a respective member of saidsecond row, and said first layer is in touching contact withsubstantially all of the outward facing surface of the first signaltransmitting portion.

By providing a cable in which only the outermost signal transmittingmembers are in contact with the first layer and recesses formed byneighbouring members of a first row accommodate members of a second row,this provides the advantage of enabling relative movement of the opticalfibres to be restrained to give the cable sufficient stiffness, whileallowing sufficient axial sliding of the optical fibres relative to eachother to minimise the application of stress to the optical fibres whenthe cable is bent.

The first signal transmitting portion may include 12 said first signaltransmitting members arranged in 4 rows having 2, 3, 4 and 3 signaltransmitting members respectively.

It is found that a cable having a signal-transmitting portion containing12 first signal-transmitting members arranged in this manner enables anoptical fibre cable having surprisingly and exceptionally favourablebending properties to be constructed.

The first signal transmitting portion may include 18 said first signaltransmitting members arranged in 5 rows having 2, 4, 5, 4 and 3 signaltransmitting members respectively.

The first signal transmitting portion may include 24 said first signaltransmitting members arranged in 5 rows having 4, 5, 6, 5 and 4 signaltransmitting members respectively.

The cable may further comprise a second signal transmitting portioncomprising a plurality of elongate, flexible second signal transmittingmembers arranged around the periphery of said first layer, wherein saidexternal dimensions of said first layer are arranged such that each saidsecond signal transmitting member is in touching contact with twoadjacent said second signal transmitting members.

The cable may further comprise a third signal transmitting portioncomprising a plurality of elongate, flexible third signal transmittingmembers arranged outwardly of said second signal transmitting portion.

The second signal transmitting members may be embedded in a secondlayer.

Said first layer may be formed of resin material cured by means ofultraviolet radiation.

An outer surface of the cable may be modified to facilitate installationinto a duct by means of fluid flow.

The outer surface may be provided with ribs.

The outer surface may include at least one anti-static material.

The outer surface may include at least one friction reducing material.

The cable may further comprise an outermost layer having an innerperiphery longer than the outer periphery of the layer adjacent theretoto enable removal of said outermost layer from the cable.

According to another aspect of the present invention, there is provideda method of forming a signal transmitting cable, the method comprising:

arranging a plurality of elongate, flexible first signal transmittingmembers in at least three rows, wherein for each said row containing aplurality of said members, said members are arranged such thatneighbouring members of a row are in touching contact with each other,and each recess formed by neighbouring members of a first said rowfacing towards a second said row accommodates a respective member of asaid second row;

surrounding said first signal transmitting members by a first layer ofresin material curable by means of radiation such that only theoutermost signal transmitting members are in contact with said firstlayer, and said first. layer is in touching contact with substantiallyall of the outward facing surface of the first signal transmittingportion; and

curing said first layer by means of radiation.

The method may further comprise arranging a plurality of elongate,flexible second signal transmitting members around the periphery of saidfirst layer such that each said second signal transmitting member is intouching contact with two adjacent said second signal transmittingmembers; and

fixing said second signal transmitting members in position relative tosaid first layer.

The step of fixing said second signal transmitting members in positionrelative to said first layer may comprise embedding said second signaltransmitting members in a second layer.

Preferred embodiments of the present invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a cable not forming partof the present invention;

FIG. 2 is a schematic cross-sectional view of cable not forming part ofthe present invention;

FIG. 3 is a schematic cross-sectional view of a cable of a firstembodiment of the present invention;

FIG. 4 is a cross-sectional view of a cable not forming part of thepresent invention;

FIG. 5 is a cross-sectional view of a cable of a second embodiment ofthe present invention; and

FIG. 6 illustrates optical attenuation characteristics of the cable ofFIG. 5 over a wide range of temperatures.

Referring to FIG. 1, a fibre optic cable 2 containing 8 optical fibresis constructed by coating a single central fibre 4 with a UV curableacrylate material 6 to increase the outside diameter of the coated fibre4 from a standard commercial diameter of 245 microns to 320 microns. Thediameter of 320 microns is such that 7 further optical fibres 10, ofidentical construction to the central fibre 4 and having a standardcommercially available diameter of 245 microns, can be arranged aroundthe circumference of the coated central fibre 4 such that each of the 7fibres 10 is in touching contact with the coated larger diameter centralfibre 4 and its two adjacent fibres 10.

The assembly is then coated with an outer layer 12 of UV curableacrylate material, the material being applied in liquid form under lowpressure. Because the 7 outer fibres 10 are in contact with the coatedcentral fibre 4 and their 2 respective neighbours 10, none of the outerfibres 10 can move during the coating process, as a result of which theacrylate material of the outer layer 12 does not penetrate into gaps 14between the coated central fibre 4 and the outer fibres 10. Thisprovides the advantage of avoiding insufficiently cured material in thegaps 14 in the assembly, which could otherwise have a detrimental effecton the optical performance of the cable.

The arrangement shown in FIG. 1 also has the advantage over thearrangement of EP 0521710 that the outside diameter of the 7 outerfibres 10 is 800 microns, while that of the prior art is 914 microns.This enables the finished cable to be smaller and the coating of thefinished cable to contain less acrylic coating material 12 than in theprior art, the acrylic coating material 12 being generally veryexpensive. Furthermore, the smaller the outside diameter of theassembly, the lower the strain applied to the outer coating 12 when thecable 2 is bent. Also, because of the circular arrangement of the outerfibres 10, the assembly has no preferential bending characteristics,which optimises the cable's performance during installation in a duct byfluid drag.

Referring to FIG. 2, in which parts common to the arrangement of FIG. 1are denoted by like reference numerals but increased by 100, a cable 102is produced by arranging 10 fibres 110 around 2 inner fibres 104 whichhave been coated with acrylate material 106 to provide an outer diameterof 547 microns. Each of the outer fibres 110 is therefore in touchingcontact with the inner layer 106 and with two adjacent outer fibres 110,as a result of which UV curable acrylate forming an outer layer 112 doesnot penetrate into the gaps 114 inwardly of the outer fibres 110.

FIG. 3 shows a first embodiment of the invention, in which parts commonto the arrangement of FIG. 1 are denoted by like reference numerals butincreased by 200. The cable 202 has a core 203 identical in constructionto the cable 2 of FIG. 1, the core 203 having an outer coating 212 ofoutside diameter of approximately 1010 microns. This enables 16 outeroptical fibres 216 to be arranged outwardly of the core 203, such thateach of the outer fibres 216 is in contact with its two neighbouringfibres 216. The entire assembly is then provided with an outer coating218 of a suitable acrylic coating to hold the outer fibres 216 in place.

It is found that as the number of layers of fibres increases, thestiffness of the assembly becomes undesirably high, as a result of whichhigh friction generated by forcing the cable around bends impedesinstallation of the cable by fluid drag. Furthermore, as the diameter ofthe cable increases, the problem of fibre breakout occurs. This problemis alleviated by replacing the outer acrylic layer 218 of the embodimentof FIG. 3 with a thin flexible lightweight sheath, which allows theouter fibres 216 to move relative to each other. Alternatively, it ispossible to encapsulate the outer fibres 216 in outer layer 218 andallow the inner fibres 210 to move relative to each other. The stiffnessof the assembly can also be adjusted by selecting suitable grades ofacrylic resin.

A further arrangement is shown in FIG. 4, in which parts common to theembodiment of FIG. 3 are denoted by like reference numerals butincreased by 100. The cable 302 has 8 fibres 304 are encapsulated by aUV curable layer 306 having a diameter of approximately 1010 microns,and 16 outer fibres 316 arranged in a circular formation around theinner fibres 304, in a manner similar to the external fibres 216 FIG. 3.

In the arrangement shown in FIG. 4, the optical fibres 304 are held inposition by means of the UV curable layer 306 so that the UV curablematerial of the layer 306 does not penetrate into the gaps between theoptical fibres 304 and the outermost optical fibres 304 are restrainedby the layer from moving axially. It is found that such an arrangementprovides surprisingly favourable bending properties, making the cableparticularly suitable for installation in a tube by means of blowing.

Exceptionally favourable bending properties are obtained in the case of12 fibres being arranged as shown in FIG. 5, in which parts common tothe embodiment of FIG. 4 are denoted by like reference numerals butincreased by 100. The cable of FIG. 5 is constructed in an identicalmanner to the cable of FIG. 4, but the inner fibres 404 of theembodiment of FIG. 5 are arranged in rows having 2, 3, 4 and 3 fibresrespectively. This cable is found to have bending properties notpreviously achievable in cables of 12 fibres. For example, the cable ofFIG. 5 meets the bending performance requirement set out in EP 0521710,although that test is designed primarily for cables containing only 4 or8 fibres. Advantageous bending properties are also achieved with cablesconstructed as in the embodiment of FIG. 5, but containing 18 fibres 404arranged in rows of 2, 4, 5, 4 and 3 fibres, and 24 fibres 404 arrangedin rows of 4, 5, 6, 5 and 4 fibres.

Referring now to FIG. 6, the signal loss over a wide temperature rangeassociated with cables of the embodiment of FIG. 5 is shown. Thedifferent curves show signal attenuation in the individual fibres 404 ofthe cable of FIG. 5. It can be seen that the cable can withstandexposure to a wide temperature range. This is a surprising result. Priorart cables as described in EP 0157610 incorporating polyethylene outerlayers display poor optical performance below approximately B20□C. Thisis usually attributed to a change of phase in polyethylene at aroundthis temperature and for this reason polyethylene is not normallyselected for the tight jacketing of fibre optic elements.

It will be appreciated by persons skilled in the art that the aboveembodiments have been described by way of example only, and not in anylimitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims.

1-27. (canceled)
 28. A signal transmitting cable comprising a firstsignal transmitting portion including a plurality of elongate, flexiblefirst signal transmitting members, wherein the first signal transmittingmembers are surrounded by a first layer of resin material curable bymeans of radiation such that only the outermost signal transmittingmembers are in contact with said first layer, and said first signaltransmitting members are arranged to form at least three rows, whereinfor each said row containing a plurality of said members, said membersare arranged such that neighbouring members of a row are in touchingcontact with each other, each recess formed by neighbouring members of afirst said row facing towards a second said row accommodates arespective member of said second row, and said first layer is intouching contact with substantially all of the outward facing surface ofthe first signal transmitting portion.
 29. A cable according to claim28, wherein the first signal transmitting portion includes 12 said firstsignal transmitting members arranged in 4 rows having 2, 3, 4 and 3signal transmitting members respectively.
 30. A cable according to claim28, wherein the first signal transmitting portion includes 18 said firstsignal transmitting members arranged in 5 rows having 2, 4, 5, 4 and 3signal transmitting members respectively.
 31. A cable according to claim28, wherein the first signal transmitting portion includes 24 said firstsignal transmitting members arranged in 5 rows having 4, 5, 6, 5 and 4signal transmitting members respectively.
 32. A cable according to claim28, wherein said first layer is formed of resin material cured by meansof ultraviolet radiation.
 33. A cable according to claim 28, furthercomprising a second signal transmitting portion comprising a pluralityof elongate, flexible second signal transmitting members arranged aroundthe periphery of said first layer, wherein said external dimensions ofsaid first layer are arranged such that each said second signaltransmitting member is in touching contact with two adjacent said secondsignal transmitting members.
 34. A cable according to claim 33, furthercomprising a third signal transmitting portion comprising a plurality ofelongate, flexible third signal transmitting members arranged outwardlyof said second signal transmitting portion.
 35. A cable according toclaim 33, wherein said second signal transmitting members are embeddedin a second layer.
 36. A cable according to claim 28, wherein an outersurface of the cable is modified to facilitate installation into a ductby means of fluid flow.
 37. A cable according to claim 36, wherein saidouter surface is provided with ribs.
 38. A cable according to claim 36,wherein said outer surface includes at least one anti-static material.39. A cable according to claim 36, wherein said outer surface includesat least one friction reducing material.
 40. A cable according to claim28, further comprising an outermost layer having an inner peripherylonger than the outer periphery of the layer adjacent thereto to enableremoval of said outermost layer from the cable.
 41. A method of forminga signal transmitting cable, the method comprising: arranging aplurality of elongate, flexible first signal transmitting members in atleast three rows, wherein for each said row containing a plurality ofsaid members, said members are arranged such that neighbouring membersof a row are in touching contact with each other, and each recess formedby neighbouring members of a first said row facing towards a second saidrow accommodates a respective member of a said second row; surroundingsaid first signal transmitting members by a first layer of resinmaterial curable by means of radiation such that only the outermostsignal transmitting layers are in contact with said first layer, andsaid first layer is in touching contact with substantially all of theoutward facing surface of the first signal transmitting portion; andcuring said first layer by means of radiation.
 42. A method according toclaim 41, further comprising: arranging a plurality of elongate,flexible second signal transmitting members around the periphery of saidfirst layer such that each said second signal transmitting member is intouching contact with two adjacent said second signal transmittingmembers; and fixing said second signal transmitting members in positionrelative to said first layer.
 43. A method according to claim 42,wherein the step of fixing said second signal transmitting members inposition relative to said first layer comprises embedding said secondsignal transmitting members in a second layer.